1. Field of the invention
The present invention is related to a projection lens system for use within a projection-type display suitable for obtaining a high-quality projected image. Particularly, it relates to a projection lens system which has a wide total field angle and a large relative aperture.
2. Description of Related Art
In recent years there has been a seen strong demand for a larger picture image on a larger screen. To satisfy the demand, a projection television is becoming popular as a method of obtaining a television image on a large screen. In a projection television, an image which is formed on a CRT for a red, a blue and a green colors, which serves as an image source, is magnified and projected by a projection lens system in order to synthesize a large size image in full color on a screen. In such an optical system, the performances of the projection lens system are important to obtain an image of high quality.
A projection lens system for use within a projection television set needs to have a relative aperture of as large as almost 1 in order to form a sufficiently bright image on a screen. At the same time, the projection lens system must ensure that aberrations are excellently corrected even at a peripheral portion of the screen. In addition, since a projection television set, being larger in depth than a direct-view television set, requires large space, demand has been mounting for a projection lens system which has a short projection distance and a large total field angle so that the size of the television set can be reduced.
The performances of a projection lens system are a decisive factor in creating a high-quality image with a video projector. The projection lens system must have: a large relative aperture to ensure brightness on a screen; a wide total field angle to attain a shortened projection distance; a reduced size and a light weight to reduce the size of the set; and an excellent focusing performance all over the screen from a center to a peripheral portion of the screen to achieve an enhanced resolution.
As a lens system which satisfies these requirements, a lens system which is constructed of glass lenses has been in wide use. However, a glass lens increases in weight when increased in the aperture. Another drawback regarding a glass lens system is that an increased number of lenses will be necessary if various aberrations are to be corrected. For these reasons, an approach employing an aspherical lens has now been widely adopted as an aberration correcting method which requires less lenses.
There are a number of examples where an aspherical plastic lens is used in order to realize a projection lens system which has a larger total field angle or a large aperture ratio. However, an aperture ratio of 1 or less can not be easily attained when a large total field angle and an excellent imaging performance all over the screen are to be achieved, and in a very likely case in which an aperture ratio of i or less is impossible, an image will not be sufficiently bright. Further, to attain excellent correction of aberrations with respect to a light flux impinging on the entire screen, the aspherical lens must have a large aspheric sag, thereby making it difficult to finish the aspherical lens with enough accuracy. Hence, full use of the design performance of the aspherical lens cannot be made, especially at the peripheral portion of the screen.
Conversely, if the aperture ratio is increased, the diameter of the light flux increases accordingly. As a result, a possibility of causing aberrations is increased especially when an aspherical plastic lens, which cannot be finished accurately in an easy manner, is disposed in the vicinity of a screen. This leads to flare-induced deterioration in the contrast of an image. In addition, it is difficult to ensure a large enough total field angle for excellent correction of aberrations at the light flux of a large diameter, which stands as an obstacle in reducing the size of the projection television set.
Further, during operation of the projection television in which the projection lens system is mounted, the temperatures of lenses become different within the projection lens system because of heat generation in the image source, i.e., the CRT. Hence, there arises a problem that the performances of the projection lens system expected during the process of designing are different from the performances observed in actual operations. This is a serious problem especially with a plastic lens since a plastic lens is susceptible to heat. One problem regarding a plastic lens due to heat generation is a change in the refractive index of the plastic lens.
With the current level of technology, a large-aperture aspherical lens made of glass is very difficult to manufacture. For this reason, an aspherical lens which is made of a plastic material is used instead. Use of a plastic aspherical lens having a large aperture in a projection lens system, however, is not advantageous since it is very difficult to form the aspherical lens. Further, the image formation performance of the aspherical lens easily deteriorates because of a change in the refractive index of the plastic material due to a change in the temperature or because of deformation of the aspherical lens due to a change in the humidity.
Further, when a large projection tube over 10 inches is used as a source of projection light to ensure brightness on the projection screen, since the lenses used to be adaptive to the large projection tube have large diameters and a converging lens having a positive converging power is thick at the center of the lens, chances are that the lenses, each increased in weight, cannot be easily assembled. If the aperture of a lens is larger than 200 mm, an excessively large thickness of the lens at the center of the lens results in difficulty in obtaining the glass material and an increase in the manufacturing cost.